Change default of auth_version to v3

Keystone v3 API is CURRENT and the v2 API is deprecated now.
So we need to change the default config of auth_version to fit for
current API status.

NOTE: Devstack changes this config to "v2" when enabling the v2 API.
      So we will still test the v2 API on the gate until we will
      change Devstack (I801e6740258ddea2a1b628a209970e0307d39d12)

Change-Id: Id5e5ed9bf4f8b0f9eb376bfc7c5801f0956da1d9

releasenotes/notes/use-keystone-v3-api-935860d30ddbb8e9.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - Tempest now defaults to using Keystone v3 API for the
+    authentication, because Keystone v3 API is CURRENT and
+    the v2 API is deprecated.

tempest/config.py

@@ -133,7 +133,7 @@ IdentityGroup = [
     cfg.StrOpt('uri_v3',
                help='Full URI of the OpenStack Identity API (Keystone), v3'),
     cfg.StrOpt('auth_version',
-               default='v2',
+               default='v3',
                help="Identity API version to be used for authentication "
                     "for API tests."),
     cfg.StrOpt('region',